1. Field of the Invention
The present invention relates to a capacitive element and a method of manufacturing the same.
2. Description of the Related Art
Currently, ABO3 perovskite type dielectric materials such as BST “(Ba,Sr)TiO3” have been studied for their applications to tunable capacitive elements in microwave and RF devices, and further to decoupling capacitors and DRAM (Dynamic Random Access Memory). For these applications, a high tunability of capacitance ΔC(V) or high capacitance density is required for dielectric materials. It is noted that the tunability of capacitance ΔC(V) can be defined as ΔC(V)=100×(Cmax−Cmin)/Cmin. Here, Cmax and Cmin are the maximum capacitance density and the minimum capacitance density over the operating voltage, respectively.
Incidentally, in order to apply a polycrystalline film with high dielectric constant such as BST to tunable capacitive elements and to elements requiring high dielectric materials, the tunability of capacitance and the permittivity thereof need to be increased while suppressing the leak current and dielectric loss of the dielectric materials.
Enhancement of the crystallinity of high dielectric constant film and optimization of its in-plane strain are considered to be important factors in order to achieve the above-described requirement.
Among these factors, the film crystallinity becomes appropriate by increasing the temperature at which the high dielectric constant film is deposited. However, in some cases, adopting a high deposition temperature results in increasing the leak current and dielectric loss of the high dielectric constant film. Moreover, titanium perovskite type dielectric materials expand greatly due to heat. For this reason, if the deposition temperature is increased, large tensile stress associated with the thermal expansion is generated in the high dielectric constant film deposited on substrates with mismatched thermal coefficients of expansion with the film. However, this leads to mismatch of lattice parameters between a base such as silicon and the high dielectric constant film, causing the problem that the electric property of the high dielectric constant film as well as the adhesion property of the film to the base are deteriorated.
Meanwhile, the in-plane strain of the film can be controlled by forming the high dielectric constant film so that the lattice parameter thereof matches with that of the base.
However, this severely restricts the kinds of bases available.
For this reason, a capacitive element having reduced in-plane strain, higher capacitance density, improved tunability of capacitance and excellent film adhesion property is demanded.
Incidentally, the leak current and reliability are improved even when amphoteric dopants are used as substitutes for either of the A-site ions or the B-site ions of the ABO3 perovskite type dielectric material. For example, Patent Document 1 discloses that Y (yttrium) is doped in BST film as shown in its FIG. 9.
However, according to the results shown in FIG. 9 of the Patent Document 1, as the doping amount of Y increases, the permittivity of the sample having a temperature of 800° C. starts to decline after showing slight increase, and the permittivity of the sample having a temperature of 750° C. shows monotonous decrease. In any samples, the permittivity is globally reduced, which is unfavorable.
Furthermore, in Patent Document 2, BST having specific composition, for example, (Ba0.85, Sr0.15)TiO3 is epitaxially grown on a Pt layer. In this way, the lattice parameter of the BST is set greater than that of the Pt, and the stress of the film is intentionally induced. This technique is supposed to provide the advantage that residual dielectric polarization is increased.
However, since the method employed in Patent Document 2 utilizes mismatch of the lattice parameters between a dielectric layer such as BST and the base, the kinds of available bases are inconveniently restricted. In addition, the BST needs to be grown epitaxially in order to utilize mismatch of the lattice parameters, so that the aforementioned advantage cannot be obtained by using other deposition methods than this method, which restricts the available method of depositing dielectric films.
Patent Documents 3 to 5 disclose other techniques relating to the present invention.
(Patent Document 1) Translated National Publication Paten Application No. Hei10-506228
(Patent Document 2) Japanese Patent NO. 2878986
(Patent Document 3) Translated National Publication Paten Application No. 2002-537627
(Patent Document 4) Japanese Patent Laid-Open NO. Hei10-27886
(Patent Document 5) Japanese Patent NO. 2681214